Recent controversy surrounding Democratic presidential candidate Hillary Clinton's health and her rival's release of his own medical information has raised questions about how much candidates must reveal about their medical history while running for office.

No U.S. law requires presidential candidates or presidents to release medical records or information about their health to the public. But 86 percent of voters say a candidate's health is important to their vote, according to a Rasmussen poll.

Most presidential candidates are middle-aged, but this issue affects somewhat younger candidates as well. John F. Kennedy, the youngest person to be elected U.S. president at 43, had Addison's disease and back pain that were treated regularly with drugs before he was elected. This information was not released to the public.

Franklin D. Roosevelt was elected president with a clear disability: He was wheelchair-bound because of polio when he was first elected, and then was re-elected three times. Late in his presidency, FDR also suffered from undisclosed congestive heart failure.

The Constitution is clear that once a president has died, the vice president takes office. But questions have arisen throughout American history about what happens when a president is incapacitated, as in a coma or under anesthesia.

Following Kennedy’s death in 1963, Congress passed the 25th Amendment to the U.S. Constitution, which said in part that when a president is incapacitated, the vice president and members of the president’s Cabinet can submit a formal declaration to the leaders of the Senate and House that the president is unable to discharge his duties. The vice president then immediately takes over.

But the amendment has never been invoked, even when former President Ronald Reagan was shot and underwent surgery in 1981.

President Barack Obama has released reports of his physical examinations periodically throughout his presidency and once during his campaign, though he was not legally required to do so. This move has become customary in the years since the passing of the 25th Amendment.

Obama's rival in 2008, Republican U.S. Senator John McCain of Arizona, released more than 1,000 pages of his medical records to the public when his age — 72 at the time — was compared to Obama's 47.